This invention pertains generally to the field of electrical heart pacemakers and control circuits therefor and more specifically to pacemakers which are adapted to operate in the atrial synchronous mode generally referred to as "VDD".
VDD pacemakers are first disclosed in U.S. Pat. No. 3,648,707 issued to Wilson Greatbatch, on Mar. 14, 1972, incorporated herein by reference in its entirety. This pacemaker included electrodes for sensing contractions of the atrium and ventricles, and a pulse generator for pacing the ventricle. After sensing a contraction of the ventricle or pacing the ventricle, a lower rate timer was restarted. If this timer expired, it triggered generation of a ventricular pacing pulse. Also included was an A-V interval timer, initiated in response to the sensing of an atrial contraction. On expiration of the A-V interval, a ventricular pacing pulse would also be triggered. The pacemaker included an upper rate timer, initiated following ventricular pacing or sensing of a ventricular contraction. During the upper rate interval, time out of the A-V interval timer would not be effective to trigger a ventricular pacing pulse. The upper rate timer allowed for inhibition of the ventricular pulse generator in the event that a natural ventricular contraction followed the sensed atrial contraction and also defined a maximum synchronous pacing rate. In other words, the pacemaker would only pace the ventricle in synchrony to the atrium up to a predetermined rate, corresponding to the upper rate interval. If the atrial rate exceeded this rate, the pacing rate would fall to 1/2 of the sensed atrial rate or to the lower rate, whichever was higher.
U.S. Pat. No. 4,059,116 issued to Adams on Jan. 12, 1976, also incorporated herein by reference in its entirety, included an improvement to the pacemaker disclosed in Greatbatch. In the Adams pacemaker, rather than blocking a ventricular stimulus in response to time out of the A-V interval during the upper rate interval, generation of a stimulus pulse was delayed until the end of the upper rate interval. In addition, a post-ventricular atrial refractory period was included, which specified a period of time following a ventricular pacing pulse or a sensed ventricular contraction, during which an atrial contraction would not be sensed. The net result of these two additional features was to produce a pacemaker which did not display such an abrupt drop in average ventricular pacing rate in response to a natural atrial rate which exceeded the rate defined by the upper rate interval. Instead, the interval between sensed atrial contractions and ventricular pacing pulses was gradually lengthened, until an atrial contraction fell within the post-ventricular atrial refractory period. The pacemaker would resynchronize on the next subsequent atrial contraction. The behavior of the Adams pacemaker resembled the natural physiologic condition known as Wenckebach syndrome. In commercially marketed pacemakers employing the Adams invention, the behavior of the pacemaker in the presence of high natural atrial rates is referred to as "Psuedo-Wenckebach" or " Pacemaker-Wenckebach" upper rate behavior.
In some heart patients, an ectopic ventricular contraction or a paced ventricular contraction not preceded by an atrial contraction may give rise to reverse conduction, back to the atrium. This is typically known as "retrograde conduction" and may produce an atrial contraction. This electrical signal accompanying such a contraction is typically referred to as a "retrograde P-wave". Retrograde P-waves typically occur within 100 to 400 ms following an ectopic ventricular contraction. If the post-ventricular atrial refractory period of the device is short enough, a retrograde P-wave may be sensed and initiate timing of the A-V interval by the pacemaker, causing generation of a subsequent ventricular pacing pulse at the expiration of the upper rate interval. If the interval between retrograde atrial depolarization and the subsequent ventricular pacing pulse is lengthy, a subsequent retrograde P-wave may be generated, reinitiating the cycle. This behavior, when it persists, is referred to as "pacemaker mediated tachycardia". Numerous improvements and modifications to dual chamber pacemakers have been suggested in order to avoid this problem.
Perhaps the simplest solution to this problem has been the provision of a lengthy post-ventricular atrial refractory period. However, the maximum tracking rate of an atrial synchronous pacemaker is limited to the rate corresponding to the post-ventricular refractory period plus the A-V interval. A 400 ms post-ventricular atrial refractory period, for example, in conjunction with a 100 ms A-V delay, would allow the pacemaker to track atrial rates only up to 120 bpm (500 ms.). In many cases, it is believed desirable to allow the pacemaker to track much higher natural atrial rates and use longer AV intervals. For example up to rates in excess of 150 bpm and AV intervals up to 200 ms. Simple lengthening of the post-ventricular atrial refractory period thus does not provide a universally acceptable solution to the problem of pacemaker mediated tachycardia.
In the context of DDD type pacemakers, a commonly employed feature directed towards eliminating pacemaker mediated tachycardia has been the lengthening of the post-ventricular atrial refractory period following a sensed natural contraction not preceded by an atrial contraction. This approach is disclosed in U.S. Pat. No. 4,407,287, issued to Herpers on Oct. 4, 1983, also incorporated herein by reference in its entirety.
In a DDD pacemaker, as disclosed in Herpers, all paced ventricular contractions are preceded by atrial contractions, either sensed or paced. However, in the context of a VDD pacemaker, another problem arises which does not display itself in the context of a DDD pacemaker. When atrial rates drop below the ventricular pacing rate set by the lower rate timer in a VDD pacemaker, the ventricular pulse generator generates pacing pulses, asynchronous to the atrium. After several pacing pulses, one pulse may be closely followed by the next subsequent bradycardic atrial contraction, which will initiate the A-V interval, and cause generation of a second, closely spaced ventricular pacing pulse at the expiration of the upper rate interval. This may initiate a pacemaker-mediated tachycardia, as discussed above.
One approach to solving this problem in the context of a VDD pacemaker is set forth in U.S. Pat. No. 4,363,325, issued to Roline et al on Dec. 14, 1982. In this case, if the atrial rate drops below a preset rate, the device switches to ventricular inhibited pacing at a second, higher rate. Returning to atrial synchronous (VDD) pacing is delayed until two successive atrial contractions at a rate higher than the ventricular inhibited rate are detected. This approach avoids the generation of closely spaced ventricular pacing pulses discussed above. However, switching to a ventricular inhibited mode prevents the pacemaker from deriving any benefit from synchronous pacing, while in VVI mode.
U.S. Pat. No. 4,344,437 issued to Markowitz on Aug. 17, 1982, represents an alternative approach to prevention of closely spaced ventricular pacing pulses in VDD pacemakers. In this device, in response to generation of a pacing pulse due to time out of the lower rate timer, the upper rate interval of the pacemaker is lengthened. This prevents an atrial bradycardia from resulting in too closely spaced ventricular pacing pulses in VDD mode. However, this approach to the problem does not address the related problem of retrograde P-waves, discussed above.
In the context of DDD pacemakers, a variety of proposals have been made to vary the post-ventricular atrial refractory period in complex fashion depending upon sensed atrial rate and sensed timing of occurrences of atrial contractions, as disclosed in U.S. Pat. No. 4,920,965 issued May 1, 1990 for a "Dual Chamber Pacemaker With Adaptive Atrial Escape Interval" and in U.S. Pat. Nos. 4,554,921 and 4,503,857 issued to Boute et al. These microprocessor based pacemakers include programs which provide for recalculation of post-ventricular atrial refractory periods, in complex fashion, based upon the above measured parameters. While these devices are believed to provide substantial useful advantages to prior art DDD pacemakers, monitoring and follow-up of the pacemakers requires telemetry of diagnostic markers to allow for determination of proper functioning.